A suspension is an important element that significantly affects the driving stability of an automobile, and typically includes, as main components thereof, a link (such as an arm, rod, etc.) that supports a wheel to be movable vertically relative to a vehicle body, a spring that is flexible to absorb impact from a road surface or the like, and a damper for damping the oscillation of the spring. Such a damper for an automotive suspension is typically embodied as a tubular damper which includes a hollow cylinder filled with hydraulic oil, a piston configured to slide in the cylinder in an axial direction, and a piston rod having an end connected with the piston, where the hydraulic oil is caused to move between multiple oil chambers in response to movement of the piston. Such a tubular damper may be of a multi-tube type or a mono-tube type.
In the tubular dampers, the piston is typically provided with a communication oil passage and a flexible valve plate, such that the valve plate provides a flow resistance to the hydraulic oil moving between the oil chambers through the communication oil passage, thereby generating a damping force. However, such a damper has constant damping characteristics, and therefore, it cannot provide a ride quality and driving stability appropriate for the road surface condition and riding circumstances. To address such a problem, there is proposed a variable damping force damper in which contraction-side and expansion-side valve plates, each made of a magnetic material, are provided on upper and lower surfaces of a piston main body, respectively, and an annular electromagnetic coil for generating a magnetic field is disposed between an outer yoke and an inner yoke that constitute the piston main body, such that the strength of the magnetic field can be varied by controlling the amount of electric current supplied to the electromagnetic coil, whereby the valve opening characteristics of the valve plates (namely, damping force) can be varied continuously (see Patent Documents 1 and 2, for example).